Typically, mixed-signal chips perform some whole function or sub-function in a larger assembly, such as the radio subsystem of a
cell phone, or the read data path and laser
SLED control logic of a
DVD player. Mixed-signal ICs often contain an entire
system-on-a-chip. They may also contain on-chip memory blocks (like
OTP), which complicates the manufacturing compared to analog ICs. A mixed-signal IC minimizes off-chip interconnects between digital and analog functionality in the system—typically reducing size and weight due to minimized packaging and a smaller
module substrate—and therefore increases the reliability of the system. Because of the use of both digital signal processing and analog circuitry, mixed-signal ICs are usually designed for a very specific purpose. Their design requires a high level of expertise and careful use of
computer aided design (CAD) tools. There also exists specific design tools (like mixed-signal simulators) or description languages (like
VHDL-AMS). Automated testing of the finished chips can also be challenging.
Teradyne,
Keysight, and
Advantest are the major suppliers of the test equipment for mixed-signal chips. There are several particular challenges of mixed-signal circuit manufacturing: •
CMOS technology is usually optimal for digital performance, while
bipolar junction transistors are usually optimal for analog performance. However, until the last decade, it was difficult to combine these cost-effectively or to design both in a single technology without serious performance compromises. The advent of technologies like high performance
CMOS,
BiCMOS, CMOS
SOI, and
SiGe have removed many of these former compromises. • Testing functional operation of mixed-signal ICs remains complex, expensive, and often is a "one-off" implementation task (meaning a lot of work is necessary for a product with a single, specific use). • Systematic design methods of analog and mixed-signal circuits are far more primitive than digital circuits. In general, analog circuit design cannot be automated to nearly the extent that digital circuit design can. Combining the two technologies multiplies this complication. • Fast-changing digital signals send noise to sensitive analog inputs. One path for this noise is
substrate coupling. A variety of techniques are used to attempt to block or cancel this noise coupling, such as
fully differential amplifiers, P+ guard-rings, differential topology, on-chip decoupling, and triple-well isolation.
Variations Mixed-signal devices are available as standard parts, but sometimes custom-designed
application-specific integrated circuits (ASICs) are necessary. ASICs are designed for new applications, when new standards emerge, or when new energy source(s) are implemented in the system. Due to their specialization, ASICs are usually only developed when production volumes are estimated to be high. The availability of ready-and-tested analog- and mixed-signal
IP blocks from foundries or dedicated design houses has lowered the gap to realize mixed-signal ASICs. There also exist mixed-signal
field-programmable gate arrays (FPGAs) and
microcontrollers. In these, the same chip that handles digital logic may contain mixed-signal structures like analog-to-digital and digital-to-analog converter(s), operational amplifiers, or wireless connectivity blocks. These mixed-signal FPGAs and microcontrollers are bridging the gap between standard mixed-signal devices, full-custom ASICs, and embedded software; they offer a solution during product development or when product volume is too low to justify an ASIC. However, they can have performance limitations, such as the resolution of the analog-to-digital converters, the speed of digital-to-analog conversion, or a limited number of inputs and outputs. Nevertheless, they can speed up the system architecture design, prototyping, and even production (at small and medium scales). Their usage also can be supported with development boards, development community, and possibly software support. ==History==